Electroplating is of great interest in many industries for many industrial purposes. Electroplating processes are used for the plating of metals on automotive parts such as bumpers and wheel rims. Sequential electroplating steps using processes containing different metals in combination with organic additives are required for many exterior and interior applications. Such metallic coatings protect the surface of the substrate from corrosion and provides a reflective, tarnish resistant finish.
Known electrolytic baths consist of an anode and a cathode, an external current source and an electrolyte salt solution within a non-conductive bath container. In use, a substrate to be plated with a metal is placed at the cathode and the metal ions in solution migrate toward the cathode under the impetus of the electrical current flowing between the anode and the cathode. Certain organic compounds are added to the electroplating bath to act as grain refining agents for the metal being plated. Such compounds also affect the brightness of the metal being plated. In nickel plating, particularly where semi-bright nickel solutions are used, an organic compound such as coumarin is often used. Saccharin is an organic compound which is often used in nickel electroplating baths for bright metal surfaces.
The current applied during electroplating is a direct current. Common metals used for plating onto substrates are transition metals such as nickel, zinc, copper and chromium.
Ongoing current feed to the electroplating bath results in a high amount of organic breakdown products in solution (due to the breakdown of the organic compounds added to the bath for obtaining semi-bright and bright finishes). Such breakdown products may alter the structure and physical properties of the plated product. Typically, the amount of organic breakdown products in the bath solution is controlled by regular and frequent use of activated carbon filtration of the bath solution.
In conventional treatment of an electroplating bath solution, activated carbon is added to the bath solution. The activated carbon may be in powder form in order to maximize the surface area available for treatment or the solution may be passed through a filter packed with activated carbon. In batch treatments the solution is removed from the electroplating bath before it is treated. In the case of the addition of powered activated carbon, mixing of the bath solution takes place and the activated carbon is allowed to settle in the bath. The bath solution is pumped from the treatment vessel and filtered. The nickel in solution complexes with the activated carbon which creates hazardous waste leading to disposal concerns. Also, the activated carbon absorbs organic breakdown products and some of the grain refining agents additives in their unreacted state. The powdered activated carbon is dusty and must be handled appropriately and safety devices must be worn by employees to insure their safety.
Electroplating processes create rinse waters which contain low levels of metal impurities. Such solution must be treated before final discharge to the environment. Various methods for treatment of wastewater from electroplating baths are used, including Advanced Oxidation Technologies (AOT). AOT is a process whereby a hydroxyl radical is generated and employed for the oxidation and thus organic compound destruction is of the organic compounds in such as waste water, for the purification of drinking water, odour control, and so on.
Fenton's reaction has been used in the treatment of such wastewater and groundwater. The use of Fenton's reaction is well known, and involves the use of hydrogen peroxide in conjunction with ferrous ions. The hydrogen peroxide is broken down into a hydroxide ion and a hydroxyl free radical. The hydroxyl radical is an oxidizing agent which can be used to oxidize and break apart organic molecules. The reaction is as follows:Fe2++H2O2→Fe3++OH−+.OH
Methods of generating hydroxyl radicals in addition to Fenton's reaction include catalytic ozonation, anodic oxidation, ultraviolet light treatment with hydrogen peroxide, chemical oxidation and titanium dioxide photocatalysis.
It is desirable to be able to reduce disposal of hazardous waste materials (solid or liquid) generated by activated carbon treatment, reduce human exposure to chemical agents and reduce the volume of bath solution needed to be treated to reduce the amount of expensive organic grain refining agents removed from the bath during treatment. It is also desirable to reduce and control the amount of organic breakdown products in the bath solution to increase the effectiveness and lifespan of the electroplating bath solution and ensure consistent physical properties and appearance of the metal being plated.
The disadvantages of the prior art may be overcome by a method and apparatus for treating an electroplating bath solution where the bath solution comprises organic compounds and a metal intended to be plated onto a substrate in the bath solution. The method in its broad aspect provides introducing an iron-containing compound into an electroplating bath solution, adjusting the pH of the bath solution and the temperature of the vessel to promote dissolution of hydrogen peroxide and generation of hydroxyl radicals, whereby the total amount of organic compounds in the bath solution is reduced.
The apparatus in its broad aspect comprises a treatment vessel for receiving the bath solution, a pump in communication with the treatment vessel for transferring a portion of the bath solution into a mixing tank for mixing and another portion into a heat exchanger for heating or cooling; another pump transfers the hydrogen peroxide to the treatment vessel whereby the bath solution is treated with iron-containing compounds and the hydrogen peroxide.
The inventors herein have unexpectedly discovered a method of treating an electroplating bath solution which comprises adjusting the pH in the range of 3.5 to 4.2 and adjusting the temperature of the bath to be in the range of 43° C. to 83° C. while adding an amount of hydrogen peroxide to the bath sufficient to promote dissolution of the hydrogen peroxide and generation of hydroxyl radicals.
In a preferred embodiment, the source of iron is ferrous sulphate and the pH is in the range of 3.7 to 4.0. The total organic carbon (TOC) is preferably reduced to a value approximately half of its original concentration. The water soluble nickel salt used for a bath for electroplating nickel is preferably selected from the group consisting of nickel sulphate, nickel chloride and nickel carbonate and mixtures thereof. Other baths having other metallic salts such as copper and zinc may also be treated in keeping with the present invention.
The method of the invention provides a number of advantages including: reducing the amount of organic breakdown products present in the electroplating bath which increases the effectiveness and lifespan of the bath, reducing downtime of the bath for maintenance, reduction of solid waste generation and handling, reduction of human exposure to potentially hazardous materials, and providing a method effective for treating less solution than the prior art thereby reducing the amount of expensive organic refining agents removed from the bath resulting in the addition of a lower amount of such refining agents for replenishment of the bath.